I recently update some Citrix servers at a customer site with SP1 for Windows 2008 R2 and ran into some problems which I blogged about earlier but I forgot one of the problems I ran into untill this morging when I saw a tweet from CitrixReadiness with the CTX129241 article about the CtxAltStr Filter Driver:

Cause

When XenApp 6.0 production version (RTW) is installed on Windows Server 2008 R2 Service Pack 1, CtxAltStr.sys is deployed in C:\Windows\SysWOW64\drivers.

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\ctxaltstr ImagePath still points to “system32\DRIVERS\CtxAltStr.sys”, therefore CtxAltStr.sys cannot be found in the Service Pack 1 environment.

Resolution

Caution! The workaround for this problem had been originally introduced in this article; modifying the pointer in the registry to the correct path in:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\ctxaltstr

From system32\DRIVERS\CtxAltStr.sys to SysWOW64\drivers\CtxAltStr.sys

If you are using this workaround, it is necessary to reset the registry value with the original path and reboot server, before you use the new resolution. If you apply the new resolution without doing that, the system may be corrupt.

Resolution1

Running the following command copies CtxAltStr.sys into the correct path:

Hotfix XA600W2K8R2X64064 and later address this issue (available for download upon request). Note that the reason why the hotfix is not publicly available is that it has not undergone the amount of regression testing required to make it publicly available.

While the issue occurs on W2k8R2 SP1, there are cases where the same issue has been observed on non SP1 environments, particularly ifKB2393802 had been applied, which seems to be causing the problem and is included in SP1. However, the Microsoft hotfix 2393802 might not be the only hotfix that exclusively causes the behavior.

I copied the file and altered the registry path to resolve this issue but Citrix now has a formal KB article so I would advise to follow one of the two explained solutions.

Kees Baggerman

Kees Baggerman is a Staff Solutions Architect for End User Computing at Nutanix. Kees has driven numerous Microsoft and Citrix, and RES infrastructures functional/technical designs, migrations, implementations engagements over the years.